In the facilities in the FA field, devices to be controlled that are control-target devices are controlled by a distributed control system. The distributed control system includes a controller unit, typically, a Programmable Logic Controller (PDC), and a remote unit connected to the controller unit via a network and located in a distributed manner (see Patent Literature 1). Generally, a plurality of remote units are installed for a single controller unit, each of which is connected to one or more devices to be controlled.
As a remote unit, a unit having one of input and output functions for a digital-signal, or a unit having one of input and output functions for an analog signal has been commonly known. Due to the characteristics that the remote units are located in a distributed manner, these remote units need to be sized smaller than the controller unit that constitutes the distributed control system, and need to operate with a minimum hardware configuration in order to reduce the overall cost of the distributed control system.
Particularly, a remote unit including an analog-signal input interface (hereinafter, “remote analog unit”) can change the type of input signal in accordance with the specifications of an analog device that is a device to be controlled, the device being supposed to be connected to the remote analog unit. Simultaneously, the remote analog unit can also change the input range. The remote analog unit has a voltage value and a current value as the type of input signal. The remote analog unit can change the type of input signal between the voltage value and the current value. The remote analog unit can change the input range to at least any of the range from −10 V to 10 V, the range from 0 V to 10 V, the range from 0 mA to 20 mA, and the range from 4 mA to 20 mA. The remote analog unit can continuously monitor a change in the input signal. Thus, increased attention has been focused on the remote analog unit as means of state monitoring or abnormality diagnosis. The remote analog unit sometimes obtains and stores therein an input signal (hereinafter, “logging”). Patent Literature 1 proposes a technique to transmit an input signal logged by a remote unit to an external computer via a PLC system.